In electrical communication systems, signal loss may be overcome by transmitting signals with a large amplitude. As signal loss problems have been compensated by different technological advancements, different solutions have focused on minimizing and/or eliminating noise and/or interference. In some applications, noise and/or interference may be minimized by high quality optical fiber communication techniques. With the development of mobile communications and the increase in demand for mobile communications, mobile phones have become commonplace. Further, with the rapid development of the Internet, a demand has developed for internet phones using VOIP (Voice over Internet Protocol) and messenger (voice) communications using personal computers.
In voice communications in both telephone networks and the Internet, telephone networks and internet networks may be linked together. In such links, however, an echo may be generated, and the generated echo may degrade the quality of communication. It may be expected that next-generation communication networks will coexist with the existing communication networks due to relatively high investment costs in establishing and maintaining communication networks. Echo cancellation factors may be a persistent issue in a telephone line that links next generation communication networks and existing communication networks. Accordingly, solutions need to be implemented that compensates for echo cancellation.
One echo cancellation factor is an echo that is generated during a telephone call. A telephone call may be placed between a subscriber of a general telephone network and an exchanger in a two wire bidirectional transmission method. A telephone path may be constructed in a transmission network between exchange offices in a four wire digital transmission method. A hybrid circuit may be used which is a matching circuit for performing conversions between a two wire transmission line and a four wire transmission line. However, an echo may be generated in the hybrid circuit by factors such as impedance mismatching. Such an echo may be a far echo and control of a far echo may improve telephone call quality.
To control an echo in a telephone communication network more efficiently, an echo signal may be precisely modeled. Characteristics of an echo path may be precisely estimated and hence an echo canceller (e.g. including an adaptive filter) may be employed. A representative algorithm used in an adaptive filter may include a least mean square (hereinafter, LMS) algorithm. An LMS algorithm may have the advantages of simplicity in computation for coefficient correction, being easily induced by a gradient tracking method, and ease in hardware implementation. Accordingly, the study of echo cancellation is often focused on using an adaptive filter employing an LMS algorithm as a controller.
In a double talk environment having a near end speaker signal s(n), the near end speaker signal may function as a barrier element in the coefficient adaptation of the adaptive filter, and its amplitude may be larger than that of an echo signal y(n). Thus, if the adaptation of filter coefficients continues in an interval where the near end speaker is present, an error signal may become relatively large, thereby causing the echo canceller employing a general LMS algorithm to diverge.
In order to compensate for this problem, a method may employ an adaptive algorithm in an echo canceller, while additionally using a double talk detector. In this method, when the detector detects double talk, the echo canceller stops adaptation and does not perform coefficient estimation anymore.
An echo cancellation method may perform sound echo cancellation in a double talk environment using an ECMLS (Expand Correlation LMS) algorithm that normally performs the estimation of an echo even in a double talk environment and has a simple structure. However, there are challenges to this method due to generation of a residual echo because the sound echo canceller using an adaptive filter cannot sufficiently estimate a sound echo path.